The LOC-RIB may have multiple paths for a prefix. The path selection mode refers to the algorithm used to decide which of these paths to advertise to an add-path peer. SR OS supports the Add-N path selection algorithm described in draft-ietf-idr-add-paths-guidelines. The Add-N algorithm selects as candidates for advertisement the N best paths with unique BGP next-hops. In the SR OS implementation, the default value of N is configurable per address family at the BGP instance, group, and neighbor levels; however, this default value can be overridden for specific prefixes using route policies. The maximum number of paths to advertise for a prefix to an add-path neighbor is the value N assigned by a BGP import policy to the best path for P; otherwise, it defaults to the neighbor, group, or instance level configuration of N for the address family to which P belongs.
Add-paths allows non-best paths to be advertised to a peer, but it still complies with basic BGP advertisement rules, such as the iBGP split horizon rule that a route learned from an iBGP neighbor cannot be readvertised to another iBGP neighbor unless the router is configured as a route reflector.